feat(back-end): add status code on each route

[AdrieneTZ]

• Add status code on each route (meet issue #10 )
• All routes have passed the thunder client test

[AdrieneTZ]

• Add a status code on each res, including checking conditions to handle the bad request.
• In the controller of register, if the email address has been registered, I'm not sure this res should end with status code 507 or 400.

[AdrieneTZ]

Thank you for reviewing this PR and giving me so many suggestions.

In the latest commit:

• Use if/ else to refactor the processes of the checking condition
• Add return before every res to make the coding style consistent